As OtheHill suggested, go to Windows Disk Management, but check whether all the disks and partitions listed at the bottom of the window have letters. Make sure the number of Volumes at the top of the window corresponds to the number of partitions at the bottom.

If it's actually been deleted and the missing letter is not the issue, and if by "the drive" you mean the partition or a flash drive, then you can try the free software called Recuva.

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